The present invention relates to a controller for a motor, and more specifically, to a controller for a brushless motor.
In a conventional method for controlling the torque of a three phase brushless motor, current values of three phases U, V, and W (phase current values) are d/q converted to generate d-axis current value and q-axis current value in accordance with a d-q coordinate system. Further, feedback control is performed based on the difference of the d-axis current value and q-axis current value from d-axis current command value and q-axis current command value (target values). The d-axis voltage command value and the q-axis voltage command value are converted back to the phase current command values of three phases U, V and W (d/q inverse conversion) to generate a motor control signal based on the phase voltage command values.
The d-q coordinate system is an orthogonal coordinate system in which the d axis extends in the same direction as the magnetic flux of a rotor in the motor and the q axis extends in a direction orthogonal to the d axis. The d/q conversion is a procedure for calculating an alternating current as a direct current by converting the vector of each phase current supplied to the brushless motor to the d-q coordinate system.
However, in practice, the phase current values of the three phases are inaccurately converted to the d-q coordinate system. Thus, control is sometimes performed with improper coordinate axes shifted from the proper or original d-q coordinate axes in the d-q coordinate system. In this case, the value of the q-axis current on the original q-axis will be lower than the q-axis current command value. This reduces the motor torque.
Accordingly, a known motor controller (Japanese Laid-Open Patent Publication No. 2001-178199) is disclosed. The controller corrects the rotation angle that is advanced during the time difference between a current detection time and a rotation angle detection time to calculate the rotation angle used in the d/q conversion based on the average angular velocity of the motor. Then, the motor controller corrects the rotation angle that is advanced during the time difference between rotation angle detection to the next rotation angle detection to calculate the rotation angle used in the d/q inverse conversion. With such a motor controller, the d/q conversion is performed with an accurate rotation angle during the current detection. Further, the output timing of the voltage command value is matched with the timing of the next rotation angle detection. The voltage command value is accurately converted to the d-q coordinate system in order to control the q-axis current with the original d-q coordinate axes. This suppresses motor torque reduction.
In the brushless motor, however, the phase of each interline current is delayed by an angle expressed with the equation φ=tan−1(ω·L/R) with respect to the phase of each interline voltage due to the influence of the motor inductance L (and armature winding resistance R). That is, the phase of the phase current is delayed by a delay angle φ with respect to the phase of the phase voltage command (a signal including the phase voltage command value).
In other words, even if the problem caused by the rotation angle advancing within the time difference of the detection timing is solved, the control is performed on d′-q′ coordinate axes shifted by the delay angle φ from the original d-q coordinate axes due to the influence of the motor inductance L, as shown in FIG. 9.
If, for example, a d-axis current command value Id* is zero, the value of the q-axis current on the original q axis, that is, the q-axis current value Iq is decreased from the q-axis current command value Iq* to Iq*cos φ. The reduction of the torque that occurs in accordance with the decreasing of the q-axis current value Iq becomes significant as the angular velocity ω increases. When the three phase brushless motor is applied to an electric power steering device (EPS), the response and feeling of the steerage during sudden operation of the steering wheel may not be satisfactory.
Further, a positive d-axis current having a d-axis current value Id equivalent to the Iq*sin φ is generated by the influence of the motor inductance L. The generation of the positive d-axis current causes heating of the motor. Further when the motor has an upper limit of power supply voltage, such as an EPS motor, as a d axis interference term increases, voltage drop caused by the influence of the motor inductance L is canceled. This restricts the flow of the q-axis current and further reduces the torque.